Aluminum and its alloys are widely used in the manufacture of many components. Often several aluminum components need to be joined to each other in a bonded manner. The aluminum components are frequently joined by utilizing a brazing process. Aluminum and aluminum alloy stock material find special utilization in the formation of components of heat exchangers such as condensers, evaporators, heater cores, coolers, and radiators. Typically, magnesium is added to the formulation of the aluminum stock material to provide the characteristics of increased strength and corrosion resistance. There are two common methods for braze joining pieces of aluminum. One method comprises controlled atmosphere brazing (CAB) wherein the brazing process is carried out in a brazing furnace employing an inert gas atmosphere of, for example, argon or nitrogen. In the CAB process a cladding material and a flux material, typically potassium fluoroaluminate, are applied to at least one of the stock pieces prior to the brazing process. The flux material breaks up the surface layer of aluminum oxide so a strong braze joint can develop. One limitation on use of the CAB process is that the maximal magnesium level in the stock material is generally limited to approximately 0.3% because of undesirable interactions between the magnesium in the stock material and the fluoride in the flux material. Therefore, it is more common to use a vacuum brazing process when the stock material contains higher levels of magnesium.
In a vacuum brazing process no flux material is employed just the cladding material is used. The vacuum brazing process can be difficult to control and requires a well-sealed furnace, careful control of pressure in the furnace, and very clean stock material.
For both the CAB and vacuum brazing processes at least one of the pieces to be joined must be clad with a thin layer of the cladding material, which actually forms the braze joint between the pieces. When joining aluminum based materials, this cladding layer typically includes aluminum as the primary component. Other materials are added to the cladding material to lower its melting point below that of the pieces to be joined. Thus, during the brazing process the cladding material is melted, flows between the pieces and then forms a solid joint when it is cooled. Typically, silicon is included in the cladding material in order to lower the melting point. In addition, the cladding material typically includes added magnesium, which acts similarly to flux in the CAB process. The magnesium diffuses during the brazing process thereby breaking up the external aluminum oxide layer, acting as a surface wetting agent. The diffusion or out-gassing of magnesium permits the cladding material to flow between the aluminum pieces and results in braze joint formation. Thus, magnesium is typically added to the cladding material for this function. The cladding material often comprises other components including calcium. Calcium is not intentionally added to the cladding material, but occurs as an impurity in the other components. It is known that calcium levels, even as low as 0.005%, can interfere with formation of a proper braze joint in a vacuum brazing process. When the calcium levels are too high the cladding material does not “wet” and flow into the joint region leading to lack of joint formation or incomplete joints. Prior to discovery of the present invention numerous steps were taken to remove calcium from the cladding material to prevent its undesirable effect on braze joint formation.